Handle missing arguments

main.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"os"
 
 	"github.com/goburrow/netforward"
 	"tailscale.com/tsnet"
@@ -26,6 +27,11 @@ func (dialer Dialer) Dial() (net.Conn, error) {
 
 func main() {
 	flag.Parse()
+	if !(host_proto != "" && hostname != "" && host_port != "" && dst_proto != "" && dst_addr != "") {
+		fmt.Println("Usage: tsnet-proxy PROTO HOSTNAME PORT DST-PROTO DST")
+		fmt.Println("Where PROTO is one of tcp, udp, unix")
+		os.Exit(1)
+	}
 
 	s := &tsnet.Server{
 		Hostname: hostname,